Former President Ford, departing from his customary support of President Carter's foreign policy, has become sharply critical of administration policies in the Middle East and southern Africa.

In the year since his defeat by Carter, the Republican former President frequently has found fault with the domestic economic policies of his Democratic successor. But he has backed Carter on the proposed Panama Canal treaties and usually has been reluctant to take issue with the administration's foreign policies, on grounds that any opposition might be misunderstood abroad.

However, in a short interview with The Washington Post, Ford said late Saturday, that he has "significant differences" with the administration on Middle East policies. He said also that administration policies were "dead in the water in souther Africa" and criticized Vice President Mondale's espousal of black majority rule in South Africa, which Ford said would make it difficult to secure South African assistance on a Rhodesian settlement.

In a White House interview last Jan. 7, Ford said the incoming administration faced three key foreign policy challenges: the Middle East, southern Africa and the pending strategic arms limitation talks (SALT) treaty.

Ford, in Houston to address a seminar of businessmen and journalists, declined in Saturday's interview to take a position on the SALT treaty beyond saying that it was "probably one of the most significant decisions that will be made by any administration in a good many years." Ford said he would be briefed both by the administration and by critics of the treaty when he visits Washington in mid-December.

The former President expressed approval of the visit of Egyptian President Anwar Sadat to Israel. He said that, the dramatic Israeli-Egyptian peace move was "risky" because of the high expectations it created, but added that the risks were worth taking in the interest of peace.

But Ford expressed "many reservations" about the Geneva conference, which the Carter administration hopes will lead to a Middle Eastern peace settlement.

"Particularly, I have reservations about the administration going to Geneva and about bringing the Soviet Union into the process at this point," Ford said.

"Our policy would have been to go to Geneva for ratification of what previously had been negotiated between the state of Israel and its Arab neighbors. That way you don't break down Geneva at the very beginning with procedural problems. If you go to Geneva without something to ratify, the whole conference could be a failure, and then there probably would be some very serious repercussions."

Ford contended that there had been no real movement during the past year toward a settlement of problems in southern Africa. He said that Mondale had made matters worse by saying "in effect that South Africa should move to one-man, one-vote overnight," which undoubtedly made [South African Prime Minister John] Vorster less than happy.

"In all honesty, you're not going to get a settlement in Rhodesia unless you get the cooperation of Mr. Vorster."

In the interview, Ford also criticized administration economic and energy policies and said that a major tax reduction is needed to stimulate the economy. The administration should "decouple" the tax reduction from pending tax revision proposals, Ford said.

The former President said it would be "premature" to say that Carter will be a one-term President, as predicted recently by House Minority Leader John J. Rhodes (R-Ariz.).

But in discussing a wide range of Republican presidential prospects for 1980, Ford said with a smile, "The more interesting question might be who will be the Democratic nominee in 1980."